1. Technical Field
The present invention relates generally to fabrication of multi-layer grids and, more particularly, to fabrication of multi-layer grids for frequency selective surfaces of an antenna.
2. Discussion
Frequency selective surfaces (FSSs) have been used in connection with wireless transmission systems such as antenna systems to reject the transmission of signals at a selected frequency band while allowing signals in a select frequency band to pass through the frequency selective surface. Accordingly, the frequency selective surface can advantageously be used to filter out signals at certain frequencies. Frequency selective surfaces are especially useful for satellite antenna systems where multiple signals at different frequencies may be present, but only selected frequency signals are to be transmitted to and from a given antenna system.
Known frequency selective surfaces have generally consisted of an array of conductive elements fabricated on a dielectric medium. The dielectric medium is generally transparent to signal radiation, while the conductive elements are configured to selectively allow signals of certain frequencies to pass through and reject signals at other frequencies. Typically, the conductive elements are often configured as closed loops, square loops, or circular loops. Generally speaking, the dimensions of the conductive elements determine the passband and rejection band of the frequency selective surface. The use of an array of conventional, single conductive loops of identical size and shapes will provide a single narrow band of rejection. However, the single loop configuration provides only limited signal rejection and a rather narrow frequency rejection band.
A double-loop frequency selective surface has also been used in connection with a dual-reflector antenna. One example of such a double-loop frequency selective surface is described in U.S. Pat. No. 5,373,302, entitled xe2x80x9cDouble-Loop Frequency Selective Surfaces for Multi Frequency Division Multiplexing in a Dual Reflector Antenna,xe2x80x9d issued to Wu on Dec. 13, 1994. The aforementioned issued U.S. patent is incorporated herein by reference. The double-loop frequency selective configuration provides an array of two different size conductive loop elements on a sub-reflector which reflects signals at two different frequency bands back into a main reflector. While dual frequency reflection bands are obtainable, each of the reflection bands effectively reflects the signals over a narrow range of frequencies.
More recently, multibeam phased array antennas have been developed especially for use on a satellite system which can be operated at various operating frequencies. For example, in a multi-band communications system, a transmit antenna may be operable to transmit signals at frequencies in the K-band such as 20.2 to 21.1 GHz, while a receive antenna may be operable to receive signals at frequencies in the Q-band such as 41 GHz. Crosslink communications among satellites may operate at frequencies in the V-band such as 62.2 GHz. One problem that may arise with the transmit antenna is that the antenna""s transmit circuit can generally employ power amplifiers which exhibit non-linear characteristics. These non-linear power amplifiers as well as other non-linear circuits which are commonly provided in active antennas may produce high frequency second and third harmonics. The high frequency second and third harmonics generated by the transmitter can interfere with the receive and crosslink channels, unless adequate signal filtering signal is provided. Such a filtering device for satellite systems and the like is generally required to be small and as light weigh as possible.
More recently, a frequency selective surface providing both signal passing and signal rejection at multiple frequency bands has been used in connection with antennas. An example of such a frequency selective surface is described in U.S. Pat. No. 5,949,387, entitled xe2x80x9cFrequency Selective Surface (FSS) Filter for an Antennaxe2x80x9d, issued to Wu et al. on Sep. 7, 1999. The aforementioned issued U.S. patent is incorporated herein by reference. The frequency selective surface filter for an antenna filters out the unwanted signals caused by the amplifier""s high frequency harmonics, especially with the transmit antenna. The frequency selective surface filter has either a single or a double conductive screen disposed on a dielectric substrate. The screen includes a grid having conductive elements which reject specific frequencies and pass other selected frequencies.
More generally, satellite antenna systems require extreme accuracy, low mass, and low costs. Such desirable qualities are often achieved by utilizing high performance multi-layer grid components, such as patch radiating arrays, polarizers, and frequency selective surface (FSS) filers. Conventionally fabricated devices, for space flight components, however, have typically used epoxy resins which often have a loss tangent of greater than 0.06. Such parts also typically utilize Kevlar(trademark) honeycomb core materials at frequencies lower than 10 GHz. At higher frequencies, such as 24.0 to 40.0 GHz, the Ka-band, 40.0 to 50.0 GHz, the Q-band, and 50.0-70.0 GHz, the V-band, the loss tangent of epoxy resins prove to be too great such that they cannot be utilized. Further, multi-layer grid components which utilize Kevlar(trademark) also exhibit very large loss tangent values and typically exhibit anisotropic properties. Thus, the assembly of multi-layer grids using epoxy resin and honeycomb core has relatively poor repeatability and provides performance inferior to what is generally desired. For example, for a V-band FSS, the wavelength is less than 0.5 cm, and the control and accuracy of layer spacing and adhesive thickness becomes critical in the fabrication process. The accuracy requirement for such layer spacing is 0.001xe2x80x3. The epoxy and honeycomb approach for building such an FSS does not meet such a high standard.
Thus, it is desirable to provide a low loss, low mass, space qualified material for fabrication of multi-layer grids and a repeatable process for fabrication of multi-layer grids.
The present invention is directed to a multi-layer grid for a frequency selective surface filter of an antenna. The multi-layer grid includes a first panel defining a first layer and further includes a first outer film and a first isotropic foam spacer bonded to the first outer film. A first adhesive layer bonds the foam spacer to the first outer film. A second outer film is bonded to the first foam spacer opposite the first outer film and a second adhesive layer bonds the foam spacer to the second outer film. The multi-layer grid further includes a second panel defining a second layer and having a third outer film and a second isotropic foam spacer bonded to the third outer film. A third adhesive layer bonds the foam spacer to the third outer film. A fourth outer film bonds to the second foam spacer opposite the third outer film, and a fourth adhesive layer bonds the foam spacer to the fourth outer film. A spacer inserted between the first and second panels integrates the first and second set of grids.
For a more complete understanding of the invention, its objects and advantages, reference should be made to the following specification and to the accompanying drawings.